// https://iai.sh.cn/problem/420
#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  double a, b, c;
  cin >> a >> b >> c;
  double gao = n * b - (n - 1) * a;
  double di = n * b - (n - 1) * c;
  double pj = (n * b - gao - di) / (n - 2);
  cout << fixed << setprecision(2) << pj;

  return 0;
}